The advancement of enhanced materials for the use of medical implants, such as joint prostheses has immensely improved the quality of life for many people over the past century. Devices such as artificial hips, knees, shoulders and other devices have allowed people who would otherwise have suffered from chronic pain and physical limitation to live active, comfortable lives. The development of such devices has confronted scientists and engineers with many technical challenges, such as in the area of materials science engineering wherein to achieve optimal implant performance various biocompatible materials with different physical and mechanical properties are bonded to each other.
Materials used for such devices must not only be non-corrosive, but must also be sufficiently resilient (having high tensile and compressive strength), and hard (having sufficient wear resistance). Since a device such as an artificial joint must undergo a great number of cycles of wear during the lifetime of the host patient, such devices must also possess great fatigue properties.
Some medical implant devices such as artificial joints must bond in some way with the patient's natural bone. Early devices employed bonding polymers, commonly referred to as bone cement to bond the implant rigidly to the anatomic structure of bone. However, more recently such devices have been constructed of porous materials such as porous Titanium (Ti) and porous Tantalum (Ta). The bone of the host patient grows into the porous material creating a strong permanent mechanical bond without the use of bone cements. Consequently, such implants are more reliable and durable in the long term than those relying on bone cement for fixation.
Such implant devices are typically manufactured from a wrought alloy, forged alloy or a powder metal injection molded process. While this produces an implant device with bulk properties that are optimized for certain overall design criteria such as biocompatibility strength and modulus of elasticity, these properties may not be optimized for property requirements specific to certain portions of the implant, such as wear or bone ingrowth characteristics.
For instance, while the use of porous materials such as porous Ti provides crucial and beneficial bonding properties, such materials may not have optimal properties in other areas. For example, porous materials may not be as hard as some other biocompatible materials and therefore may not have acceptable wear properties. However, because of the overriding importance of strong permanent bonding with the host patient bone, such porous materials have continued to be used in spite of less than optimal wear properties.
In order to enhance the wear properties of a device such as an artificial joint, prior art devices have been constructed in more than one piece. A first portion of the joint implant, that which will bond to the bone, has typically been constructed of a porous material such as porous titanium, and a second piece, such as the bearing surface of the joint has been constructed of a much harder, more wear resistant material such as alloys of cobalt and chrome (Co—Cr). The first and second pieces are then bonded together in an attempt to obtain the benefits of both materials. One challenge to using such a technique is that of achieving a sufficiently strong, permanent bond between the first and second portions, without the use of adhesives that may be biologically incompatible or may fail under the stresses imposed by the body of the patient. Attempting to weld such materials together can cause the non-porous material to flow into the porous material, destroying the porosity of the porous material and degrading the ability of the device to bond with the patient's bone. In addition, such materials, being dissimilar metals, often experience galvanic corrosion when bonded together in such a manner.
Therefore, there remains need for a device (and method for making the same) such as an artificial joint which can take advantage of the properties of a first material, such as the porosity of porous Ta or Ti, and also take advantage of the properties of a second material, such as the hardness of a material like Co—Cr, for use in a bearing environment such as a ball or socket of a joint. Such a device would preferably not exhibit any delamination between the two materials and would not experience any galvanic corrosion. Such a device would also preferably not diminish the porosity of the porous material due to the flow of the other material thereinto.